The relationship between the water regime and morphology of soils in the Weatherley catchment, northerly Eastern Cape

[摘要] English: The Weatherley catchment is situated close to Maclear in the northern Eastern CapeProvince. Soil water contents have been measured regularly, as part of a long-termproject, at 28 sites in the catchment for six years to quantify the soil water regime.For this study tEm of these sites were selected, described and analysed in detail tocharacterize the soil properties. This data was used to determine the relationshipbetween soil water regime and soil profile morphology.A correlation was found to exist between mean annual duration of water saturationabove 0.7 of porosity (ADs>o.7, days year) and other soil properties. CECsoilincreases from 3.5 to 11.5 crnol, kg-1, base saturation increases from 25 to 44 %,and the underlying unspecified material with signs of wetness or G horizons occurcloser to the soil's surface as mean ADs>o.7 increases.Orthic A horizons that occur in seeps are wetter, with mean ADs>o.7 212 - 365 daysyear, than orthic A horizons that occur on ridges with mean ADs>o.7 of < 25 daysyear. The former have common and many Fe oxide mottles, whereas the latterlack clear mottling. ADs>o.7 data.shows that orthic A horizons overlying red apedal B,yellow-brown apedal B or neocutanic B horizons are well drained, while thoseoverlying E, soft plinthic Band G horizons are wetter. It is proposed that orthic Ahorizons can be subdivided into wetness classes using the occurrence of mottles, aswell as the nature of the underlying horizon.The Longlands (P201 and P207) and Westleigh (P204) soils, characterized by theoccurrence of a soft plinthic B horizon, have intermediate mean ADs>o.7. Theyincrease from 84 to 157 days year in orthic A horizons, to 231 days year in softplinthic B horizons and 360 days year in the underlying unspecified material withsigns of wetness. The chromatic soils, including Pinedene (P202), Tukulu (P203)and Bloemdal (P210) soil forms, defined as having yellow-brown apedal B,neocutanic B or red apedal B horizons as the second horizon in the profilesrespectively have the driest water regime of the soils studied. The A and B1 horizons have mean ADs>o.7 < 40 days year', the B2 horizons 88 days year, and theunderlying unspecified material with signs of wetness 130 to 350 days year .Pooling the data from all the profiles studied produces the following mean ADs>o.7 forthe different diagnostic horizons: G horizons = 324 ± 20 days year: unspecifiedmaterial with signs of wetness = 243 ± 40 days year: E horizons = 163 ± 56 daysyear: soft plinthic B horizons = 148 ± 45 days year: orthic A horizons = 144 ± 40days year'; and chromatic (red apedal B, yellow-brown apedal Band neocutanic B)horizons = 88 ± 51 days year. There is an association between mean ADs>o.7 andother soil properties. For example clay content, CEC, basic cations and pH arehighest in G horizons and lowest in chromatic horizons. CBD extractable Fe and Mnare found to be roughly twice as much in soft plinthic B, chromatic B and unspecifiedmaterial with signs of wetness horizons compared to orthic A, E or G horizons.Digital soil horizon photographs were quantified and classified into diagnostic red,yellow-brown, grey and black colours, using the Red and Green values of RGBnotationin ArcView SpatialAnalyst. This methodology produced promising results, asproven by classification of photographed Munsell colour sheets. Colour classificationof horizon photographs generally supports the field colour classification of thesehorizons using Munsell colour sheets. In general, the profiles described for this studyare dominated by diagnostic grey and yellow-brown colours, with the exception of theBloemdal (P210) which is diagnostic red.

[发布机构] University of the Free State